Selectivity in Radial Systems

Selectivity between series-con- nected fuses

The incoming feeder lines and the outgoing feeders of the busbar of a distribution board carry different op- erating currents and, therefore, also have different cross-sections. Conse- quently, they are usually protected by fuses with different rated currents which ensure selectivity on account of the different operating behavior.

Selectivity between series-con- nected fuses with identical utiliza- tion categories

When fuses of the same utilization category (e.g. gL or gG) are used, se- lectivity is ensured across the entire overcurrent range up to the rated breaking capacity (absolute selectiv- ity) if the rated currents differ by a factor of 1.6 or higher (Fig. 3/20). The Joulean heat values (I2_t-values)

should be compared in case of high short-circuit currents. In the example shown, a 160 A LV HRC fuse would also have absolute selectivity with respect to a 100 A LV HRC fuse.

Selectivity between series- connected circuit-breakers

Selectivity by grading the operat- ing currents of instantaneous over- current releases

(current grading)

Selectivity can be achieved by grad- ing the operating currents of instanta- neous overcurrent releases (I-re-

leases) (Fig. 3/21). Prerequisites for this are:

Current grading with different short-circuit currents

The short-circuit currents in the event of a short circuit at the respective lo- cations of the circuit-breakers are sufficiently different.

Current grading with differently configured I-releases

The rated currents and, therefore, the

I-release values of the upstream and

downstream circuit-breakers differ accordingly.

5-second breaking and line- protection conditions

In complying with the 5-second breaking condition specified in HD 384.4.41 / IEC 60364-4-41 / DIN VDE 0100-410 or the 5-second line-protection condition specified in DIN VDE 0100-430 (if line protection cannot be provided in any other way),

the I-release must generally be set to 4,000 A so that even very small short circuits are cleared at the input termi- nals of the downstream circuit- breaker Q1 within the specified time. Only partial selectivity can be estab- lished by comparing characteristic curves for current grading since the increased appearance of broken lines in the curve in the range < 100 ms, which result from the complicated dy- namic switching and tripping opera- tions, does not permit conclusions to be drawn with regard to selectivity.

Possible solution: dynamic selectivity

Selectivity through circuit-breaker coordination (dynamic selectivity)

With high-speed operations, e.g. in the event of a short circuit, and the in- teraction of series-connected protec- tion devices, the dynamic processes in the circuit and in the electro- mechanical releases have a consider- able effect on selectivity behavior, particularly if current limiters are used. 1.37 s ts I 200 A (160 A) Ik =1300 A 50 A 50 A 100 A Ik =1300 A 101 ₁₀2 ₁₀3 ₁₀4 1.3 _[A] 100 A size 00 200 A size 1 [s] 1.4 0.03 K1 a) Selective isolation of short circuit K1

b) Prearcing times where Ik =1300 A

Fig. 3/20 Selectivity between series-connected LV HRC fuses with identical utilization categories (example)

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Selectivity is also achieved if the downstream current-limiting protec- tion device trips so quickly that, al- though the let-through current does momentarily exceed the operating value of the upstream protection de- vice, the ”mechanically slow” re- lease does not have time to trigger. The let-through current depends on the peak short-circuit current and cur- rent limiting characteristics.

Selectivity limits of two series-con- nected circuit-breakers

A maximum short-circuit value – the selectivity limit – up to which the downstream circuit-breaker can open more quickly and alone, i.e. selec- tively, can be determined for each switchgear assembly.

Table 3/16 shows an example of a se- lectivity table. The selectivity limit indi- cated in the table may be well above the operating value of the instanta- neous overcurrent release in the up- stream circuit-breaker (see Fig. 3/22). Irrespective of this, it is important to check the selectivity in the event of an overload by comparing the charac- teristic curves and by means of trip- ping times in accordance with the relevant regulations.

Generally speaking, only partial selec- tivity is possible in the case of dy- namic selectivity with short circuits. This may be sufficient (full selectivity) if the prospective maximum short-cir- cuit current at the downstream pro- tective device is lower than the es- tablished selectivity limit.

With partial selectivity, which usually arises with current grading owing to the clearance condition (see Fig. 3/20), consideration of dynamic selec-

tivity provides a suitable possibility for establishing full selectivity without having to use switchgear with short- time-delay overcurrent releases.

Selectivity by means of short- time-delay overcurrent releases (time grading)

Time grading by short-time-delay releases

If current grading is not possible on account of the requirements listed on page 36 and cannot be achieved by selecting the switchgear in accordance with the selectivity tables (dynamic se- lectivity), selectivity can be provided by time grading short-time delay overcur- rent releases. This requires grading of both the tripping delays and the appropriate operating currents. 5 4 102 _{2 5 10}3 _{2 5 10}4 _{2 5} [A] Current I 10-2 10-1 100 101 102 103 104 Opening time t 102 101 100 min. [s] I II I (720 A) I (4000 A) I (6000 A)1) L Q2 L Q1 M 3~ Ir = 600 A (L-release) 4000 A (I-release) Ie = Ir = 60 A (L-release) 720 A (I-release) Ii = Sr = 400 kVA at 400 V, 50 Hz Ukr = 4% Ir = 577 A Ik≈ 15 KA Q2 Ik = 10 kA 4.8 kA 2.1 kA Q1 II I a) Single-line diagram

Q1 Circuit-breaker for motor protection (current-limiting)

Q2 Circuit-breaker (zero-current interrupter)

b) Tripping curves

L Inverse-time delay overload release

I Instantaneous electromagnetic overcurrent release

1) _{Maximum setting range}

Time grading with virtually identical short-circuit currents

The upstream circuit-breaker is equipped with short-time-delay over- current releases (S) so that, if a fault occurs, only the downstream circuit- breaker disconnects the affected part of the installation from the system.

Time grading can be implemented to safeguard selectivity if the prospec- tive short-circuit currents are almost identical. This requires grading of both the tripping delays and the operating currents of the overcurrent releases.

In addition to the diagram with the four series-connected circuit-break- ers, Fig. 3/22 also contains the associ- ated grading diagram. The necessary grading time, which allows for all scatter bands, depends on the operat- ing principle of the release and the type of circuit-breaker.

Electronic S-releases

With electronic short-time-delay overcurrent releases (S-releases), a grading time of approximately 70 ms to 100 ms from circuit-breaker to cir- cuit-breaker is sufficient to allow for all scatter bands.

Operating current

The operating current of the short- time-delay overcurrent release should be set to at least 1.45 times (twice per 20% scatter, unless other values are specified by the manufacturer) the value of the downstream circuit- breaker.

Additional I-releases

In order to reduce the short-circuit stress in the event of a ”dead” short circuit at the upstream circuit-break- ers, they can be fitted with instanta- neous electromagnetic overcurrent releases in addition to the short-time delay releases (Fig. 3/23). The value selected for the operating current of the instantaneous electromagnetic overcurrent releases must be high enough to ensure that the releases only operate in case of direct ”dead” short circuits and, under normal oper- ating conditions, do not interfere with

Zone-selective interlocking (ZSI)

A microprocessor-controlled short- time grading control, also called “zone-selective interlocking”, has been developed for circuit-breakers to prevent excessively long tripping times when several circuit-breakers are connected in series. This control function allows the tripping delay to be reduced to max. 50 ms for the cir- cuit-breakers located upstream of the short circuit.

The method of operation regarding zone-selective interlocking is illus- trated in Fig. 3/24. A short circuit at K1 is detected by Q1, Q3, and Q5. If ZSI is active, Q3 is temporarily dis- abled by Q1 and Q5 by Q3 by means of appropriate communication lines. Since Q1 does not receive any dis- abling signal, it trips after only 10 ms. A short circuit at K2 is only detected by Q5; since it does not receive any disabling signal, it trips after only 50 ms. Without "ZSI", tripping would only occur after 150 ms.

Selectivity between circuit- breaker and fuse

When considering selectivity in con- junction with fuses, a permissible scatter band of ± 10% in the direction of current flow must be allowed for in the time-current characteristics.

M Power supply system Circuit- breaker 3WL1 3WL1 3VL 3VL 3VL 3RV Instantaneous 80 ms 150 ms 220 ms Delay time td of S-release

Fig. 3/22 Required delay time settings for electromagnetic short-time-delay S-releases for selective short-cir- cuit protection

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tZSi = 50 ms I Current Opening time t td = 150 ms Q1/Q2 Q3/Q4 Q5 td = 150 ms td = 80 ms tZSi td = 10 ms Icn 104 ₁₀5 103 102 10-2 10-1 100 101 102 103 104 Q5 tZSi = 50 ms K2 td = 80 ms Q3 Q4 tZSi = td td = 10 ms Q1 Q2 tZSi = td td =10 ms K1 Communication lines A E A E A E A E A E [A] [s] I 102 _{2 5 10}3 _{2 5 10}4 _{2 5} [A] Current 10-2 10-1 100 101 102 103 104 Opening time t [s] L Q2 Q1 Sr = 1000 kVA at 400 V, 50 Hz Ukr = 6% In = 1445 A Ik≈ 24.1 kA 105 Main distribution board Sub- distribution board Ik = 10 kA Ik = 17 kA td2 = 80 ms td3 = 150 ms n (20 kA) Q3 Q2 Q1 M_~ L L Q3 S S td2 = 80 ms td3 = 150 ms n

Fig. 3/23 Selectivity between three series-connected circuit-breakers with limitation of short-circuit stress by means of an additional I-release in circuit-breaker Q3

Circuit-breaker with downstream fuse

Selectivity between LI-releases and fuses with very low rated currents

In the overload range up to the oper- ating current Iiof the delayed overcur-

rent release, partial selectivity is achieved if the upper scatter band of the fuse characteristic does not touch

the tripping characteristic of the fully preloaded instantaneous overcurrent release and maintains a safety margin of tA≥1 s (Fig. 3/25).

A reduction in the tripping time of up to 25% must be allowed for at normal operating temperatures (unless the manufacturer states otherwise).

Absolute selectivity for circuit-break- ers without short-time-delay overcur- rent releases is achieved if the let- through current of the fuse IDdoes

not reach the operating current of the instantaneous overcurrent release (please refer to current limiting dia- gram for LV HRC fuses in ”Electrical Installations Handbook”, Section 4.1.1). This is, however, only to be expected for a fuse, the rated current of which is very low compared with the rated continuous current.

Selectivity ratios between LS-re- leases and fuses with relatively high rated currents

Due to the dynamic processes that take place in electromagnetic re- leases, absolute selectivity can also be achieved with fuses, whose ID

briefly exceeds the operating current. Once again, selectivity can only be verified by means of appropriate measurements of Ii. Absolute selec-

tivity can be achieved by using circuit- breakers with short-time-delay over- current releases (S-releases) if the safety margin for the operating cur- rent tdbetween the upper scatter

band of the fuse characteristic and the delay time of the S-release tdis

selected so that tA≥100 ms (Fig.

3/26).

Selectivity between fuse and downstream circuit-breaker

Selectivity ratios in the overload range

In order to achieve selectivity in the overload range, a safety margin of

tA≥1 s is required between the lower

scatter band of the fuse and the char- acteristic curve of the inverse-time- delay overload release (Fig. 3/27). t Ii F1 I F1 Q1 L I I Q1 tA I I Overload range F1 Q1 L I tA Ii

The time-current characteristics (scatter bands) do not touch

Fuse Circuit-breaker Inverse-time-delay overload delay Instantaneous electromagnetic overcurrent release Safety margin

Operating current of n release L t Id F1 S F1 Q1 L S Ik Q1 ts I L L S tA Id ts td Overload release

Short-time-delay overcurrent release Safety margin

Operating current of s release Prearcing time of fuse Delay time of s release

tA≥ 100 ms

td

Fig. 3/25 Selectivity between circuit-breaker and downstream fuse in overload range

Fig. 3/26 Selectivity between circuit-breaker with LS-releases and downstream fuse; short-circuit current range

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In case of short circuits, it is impor- tant to remember that, after the re- leases in the circuit-breaker have tripped, the fuse continues to be heated during the arcing time. The se- lectivity limit lies approximately at the point where a safety margin of 70 ms between the lower scatter band of the fuse and the operating time of the instantaneous overcurrent release or the delay time of the short-time-delay overcurrent release is undershot.

Short-circuit range

A reliable and usually relatively high selectivity limit for the short-circuit range can be determined in the

I2_{t- diagram. In this diagram, the maxi-}

mum let-through I2_{t value of the cir-} cuit-breaker is compared with the minimum prearcing I2_{t value of the} fuse (Fig. 2/28). Since these values are maximum and minimum values, the scatter bands are not necessary.

t F1 I F1 Q1 L I I Q1 I L tA≥ 1 s Overload range I F1 Q1 L I tA Ii

The time-current characteristics (scatter bands) do not touch

Fuse Circuit-breaker Inverse-time-delay overload release Instantaneous electromagnetic overcurrent release Safety margin

Operating current of I-release

I2_t F1 F1 Q1 L I Ik Q1 I Q1 F1 ISel Circuit-breaker (max. let-through value) Fuse (min. prearcing value) Selectivity limit

Selectivity range

ISel

Fig. 3/27 Selectivity between fuse and downstream circuit-breaker; overload range

Fig. 3/28 Selectivity between fuse and downstream circuit-breaker; short circuit

I 102 2 104 102 10-1 100 101 102 103 104 t [s] L Q2 Q1 Ir = 600 A Isd = 3,000 A Ik≤ 10 kA Q2 Q1 M ~ L L S L S Ik≤ 10 kA Q3L S I T1 Equal ratings T2 Ik Part L I Ir = 200 A Ii = 2,400 A IkΣ 4 6 tö1 103 _{2 4} 3 6 2 4 [A] Ik Part ttd2/3 ≈150 ms (≥ 70 ms) Q2+Q3 Separate Parallel Base IkΣ Ii

Fig. 3/29 Selectivity with two infeed transformers of the same rating and operating simultaneously. Example with outgoing feeder in the center of the busbars.

Selectivity with parallel infeeds

Improving selectivity with parallel infeeds

With parallel infeeds to a busbar, the total short-circuit current IK∑that oc-

curs in the faulted outgoing feeder comprises the partial short-circuit cur- rents Ik Partin the individual infeeds

and represents the base current in the grading diagram (Fig. 3/29). This is the case for all fault types.

Two identical infeeds

If a short circuit occurs in the outgo- ing feeder downstream of the circuit- breaker Q1, the total short-circuit current Ik∑of e.g. ≤20 kA flows via

this breaker, while the infeed circuit- breakers Q2 and Q3, with the out- going feeder connected centrally to the busbars and incoming feeders of equal length, each carry only half this current, i.e. ≤10 kA.

Parallel operation permits addi- tional current selectivity by means of a shift in the tripping curve (Ii) of the LS-releases of the infeed cir- cuit-breaker

Additional current selectivity with parallel transformer operation

In the grading diagram, the tripping curve of circuit-breakers Q2 and Q3 must, therefore, be considered in relation to the base current of the circuit-breaker Q1.

Characteristic displacement factor

Since the total short-circuit current is ideally distributed equally among the two infeeds (ignoring the load cur- rents in the other outgoing feeders) with the outgoing feeder located at the center of the busbars, the tripping curve of circuit-breakers Q2 and Q3 can be shifted optimally to the right along the current scale by a character- istic displacement factor of 2 up to the line Ik∑, which represents the

base current for this fault condition. The result of this is selectivity both with regard to time and also current. If the characteristic curve of the indi- vidual circuit-breaker is used instead of the shifted characteristic, the exact short-circuit current (distribution) which flows through the circuit- breaker must be taken into considera- tion.

With asymmetrical configurations and with infeeds and outgoing feeders located at the busbars, short-circuit current distribution will differ accord- ing to the impedance ratio along the incoming feeder lines.

Reduced selectivity with LV HRC fuses with a rating of 630 to 1,000 A near an infeed

This is particularly significant in the event of fused outgoing circuits with high current ratings, e.g. 630 to 1,000 A. It is important to ensure that a safety margin of ≥100 ms between the tripping characteristic of the S-re- lease and the prearcing-time/current characteristic of the LV HRC fuse is provided not only with parallel opera- tion, but also with individual trans- former operation.

When setting the releases of circuit- breakers Q1, Q2 and Q3, it must be ensured that selectivity is also achieved for operation with one transformer and for all short-circuit currents (single- to three-phase). For cost-related reasons, S-releases for the feeder circuit-breakers must also be provided for low and medium rated fuse currents as the resulting current selectivity of I-releases is insufficient.

Three identical infeeds

With parallel operation of three trans- formers, the selectivity ratios will, owing to the additional current selec- tivity, be more favorable than with two units since the characteristic dis- placement factor is > 2 and < 3. Once again, LS-releases are required for the circuit-breakers in the infeeds in order to achieve unequivocal selec- tivity ratios. Ik < 15 kA Ik < 15 kA 15 kA IkΣ < 30 kA Q1LS I T1 Q2 Q3 T2 T3 15 kA Ik Part 1 Q1 Q2 Q3 Ik Part 2 IkΣ

Fig. 3/30 Selectivity with three infeed trans- formers operating simultaneously

Fig. 3/31 Short-circuit distribution via the tie breaker Q3 with two infeeds Q1 and Q2

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In document 53534403 Siemens Switchboard and Protection Manual (Page 81-88)